1. Field of the Invention
This invention concerns a spin-valve type magnetoresistance sensor in which a free ferromagnetic layer and pinned ferromagnetic layer enclosing a nonmagnetic spacer layer on substrate, and in which the magnetization direction of the pinned ferromagnetic layer is pinned by an antiferromagnetic layer, as well as a thin film magnetic head provided with a spin-valve magnetoresistance sensor, for use in magnetic recording devices.
2. Background Information
In the past, magnetoresistance (MR) sensors have been developed with a spin-valve film structure exhibiting the giant magnetoresistance effect in order to raise the magnetic field sensitivity of reproduction magnetic heads. In general, spin-valve MR films consist of a sandwich structure in which two magnetic layers enclose a nonmagnetic spacer layer on substrate; one of these, the pinned layer (fixed ferromagnetic layer), has its magnetization fixed parallel to the signal magnetic field by the exchange coupling magnetic field with the adjacent antiferromagnetic layer, and the magnetization of the other, free layer (free ferromagnetic layer) has a single magnetic domain induced by a hard-bias method using the magnetic field of a permanent magnet, so that its magnetization rotates freely under the action of an external magnetic field. When the magnetization of the free layer rotates under the action of the external magnetic field from a magnetic recording medium or other source, the angular difference in magnetization direction arising between the two magnetic layers causes a change in the resistance of the MR film. By means of this magnetoresistive change, the data recorded in the recording medium can be detected.
On the other hand, in order to enhance the magnetic sensitivity of a spin-valve MR sensor, it is effective to reduce the film thickness of the free layer; but it is known that if the free layer thickness is reduced too much, for example so that the mean free path of conduction electrons is reduced to 30 to 50 xc3x85 or so, then the magnetoresistance change (MR ratio) declines. Recently, in order to resolve this problem, methods have been studied for raising the MR ratio by forming a back layer (or backing layer) of a nonmagnetic metal layer adjacent to the free layer on the opposite side of the nonmagnetic spacer layer so as to effectively increase the mean free path of conduction electrons, as disclosed for example in the specification of patent U.S. Pat. No. 2,744,883.
In a paper by H. Iwasaki et al titled xe2x80x9cSpin Filter Spin Valve Heads with Ultrathin CoFe Free Layersxe2x80x9d (IEEE, Intermag 99, BA-04 (1999)), a spin-valve film with a spin filter structure provided with a free layer comprising a CoFe thin film in contact with a highly conductive layer on the side opposite the Cu spacer layer, as well as a magnetic head using this, were proposed. It is reported that by this means, the mean free path of up-spin electrons is improved by the existence of the highly conductive layer, and moreover the mean free path between up-spin electrons and down-spin electrons is maintained, so that a high and stable MR ratio is obtained even for an extremely thin free layer of thickness 15 xc3x85, and consequently sensor sensitivity is improved, and high recording densities can be realized.
However, when the characteristics of spin-valve films with such a spin filter structure were actually measured with the back layer thickness varied, it was found that the interlayer coupling magnetic field (Hint) acting between the free layer and the pinned layer fluctuates greatly with the back layer thickness.
FIG. 5 shows the configuration of the spin-valve films used in these measurements. On top of an underlayer consisting of a Ta (30 xc3x85) film 2 and NiFeCr (40 xc3x85) film 3 formed on a substrate 1 are formed a PtMn (250 xc3x85) antiferromagnetic layer 4; a synthetic-structure pinned layer consisting of a CoFe (20 xc3x85) film 5, Ru (8.5 xc3x85) film 6, and CoFe (26 xc3x85) film 7; a Cu (24 xc3x85) nonmagnetic spacer layer 8; a free layer consisting of a CoFe (10 xc3x85) film 9 and NiFe (20 xc3x85) film 10; and a Cu nonmagnetic metal layer 11 as a back layer. On top of this is formed a Ta (30 xc3x85) protective layer 12. After film formation, heat treatment is performed for 10 hours at 270xc2x0 C. in a 15 kG magnetic field in vacuum, in order to render the PtMn antiferromagnetic layer 4 regular and to induce an exchange coupling with the aforementioned pinned layer.
FIG. 6 shows changes in Hint for this spin-valve film as the thickness t of the Cu nonmagnetic metal layer 11 was varied from 0 to 40 xc3x85. In the figure, when the Cu film thickness t was increased from 5 xc3x85 to 15 xc3x85, Hint decreased from 8 Oe to 2 Oe. Because the average amount of change per Angstrom is 0.6 Oe/xc3x85, even if, for example, the back layer film thickness could be controlled with a precision of xc2x11 xc3x85, the amount of change in Hint in this error range would be as great as 1.2 Oe, indicating a large dependence on the back layer film thickness.
In particular, Hint is an important parameter affecting the nonlinearity (asymmetry) of the sensor output; hence scattering in Hint directly causes scattering in the sensor performance, and stability suffers. For this reason, when such a sensor is applied in a read magnetic head, scattering occurs in the magnetic transducing characteristics due to manufacturing conditions, production yields decrease, and reliability is degraded.
A spin-valve magnetoresistance sensor is disclosed. In one embodiment, the spin-valve magnetoresistance sensor is characterized by being provided with a free-side ferromagnetic layer deposited on substrate, a pinned-side ferromagnetic layer, a nonmagnetic spacer layer enclosed between both aforementioned ferromagnetic layers, an antiferromagnetic layer adjacent to the aforementioned pinned-side ferromagnetic layer and which pins said fixed-side ferromagnetic layer, and a back layer comprising at least two nonmagnetic metal layers deposited on the side opposite the aforementioned nonmagnetic spacer layer and adjacent to the aforementioned free-side ferromagnetic layer.